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Heat Pumps Are Weird


Brian G
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Yo George,

I have a couple of heat pump questions for you.

1. What (if any) output temperature would you consider to be the "minimum acceptable" for a heat pump?

2. What is the difference beween "auxiliary heat" and "emergency heat"?

I was inspecting one the other day, and checking the output temp just as I always do on gas or resistive heat. It was only going up to about 96. The "auxiliary heat" light was on at the thermostat at the time. I thought "What the hell, let's see what we get on "emergency heat", and flipped it on at the thermostat. Within a couple of minutes the temp was down to 83 and dropping...Huh?

I admit it, this is not my strong suit. What's the story George? Does this sound like a deficiency in the machine or the man?

Brian G.

Surely It Is Not I! [:-boggled]

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Brian,

Here in Michigan, where we wear our winter coats 9 months a year, heat pumps are very rare.

My guess is however that the output temperature is not at all important, it is the temperature rise. I would measure that and compare it with the MFG rating plate. Output is directly related to input. A 10 degree difference in ambient return should equal a 9.5 degree difference in output. Plus, increasing or decreasing the CFM across the coil will affect both TR and output reading's

As far as auxiliary and emergency, in this application, I expect that they may be interchangeable. If there is a difference it seems that auxiliary is a booster heat like an electric heating element or a burner that kicks in when the heat load is too great for the heat pump alone. Emergency is something that kicks in, in the event of a primary equipment malfunction. The best person I can think of to kick these questions to is Mr. Cramer

George

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Originally posted by Brian G.

Yo George,

I have a couple of heat pump questions for you.

1. What (if any) output temperature would you consider to be the "minimum acceptable" for a heat pump?

It depends on a lot of different factors. First of all, as George said, you’ve got to look at the change in temperature across the indoor coil, not just the output. Next, you have to understand that the expected temperature change will be different depending on the outdoor temperature. Also, if the filter or coil is dirty or if there are restrictions in the ductwork (supply or return) the differential will be artificially high. If you take all that into consideration, then check out the attached chart. I believe I stole it from Jay Walker. It's worked well for me. I'm not sure it'll work as well down near the equator where you live.

2. What is the difference beween "auxiliary heat" and "emergency heat"?

What George said.

I was inspecting one the other day, and checking the output temp just as I always do on gas or resistive heat. It was only going up to about 96. The "auxiliary heat" light was on at the thermostat at the time. I thought "What the hell, let's see what we get on "emergency heat", and flipped it on at the thermostat. Within a couple of minutes the temp was down to 83 and dropping...Huh?

Depending on the outdoor temperature and the ambient indoor temperature, 96 could be just fine. If the auxiliary heat light was on, it means that the stat was calling for auxiliary heat, not necessarily that it was actually firing. It also means that you probably had set it for more than two degrees above the indoor ambient temperature. (Doing this automatically brings on the auxiliary.) When you flipped it to "emergency" you told it to stop running the heat pump and only run the heat strips (I'm assuming that this had electric back up and not fossil fuel?). Depending on the stat, there could have been a built-in delay between modes. So you may have been watching the system just cooling down between modes. In general, you don't want to be switching modes while the unit is running. Give it a couple of minutes to rest between modes.

I admit it, this is not my strong suit. What's the story George? Does this sound like a deficiency in the machine or the man?

Well, I can't speak for George, but really. . . is there a difference? Aren't we all just machines, sinew and blood, bile and breath? Is there not a little of the heat pump in us all? And if so, what is the measure by which we shall be judged? Our "temperature rise" as it were.

- Jim Katen, Oregon

Download Attachment: icon_adobe.gif Heat Pump Splits.pdf

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Brian,

Oh yea, Or Jim Kated.

Jim,

I do not understand how outdoor temperature could affect indoor temperature rise, unless the equipment is staged or augmented using an outdoor sensor to control the process. But in a pure heat pump application, ODT should have no bearing on TR across the coil.

If my indoor return air is 65 degrees Wether it is 40 degrees outdoors or 30 degrees, I should have the same rise across the coil. The only difference ODT makes would be in run time. Perhaps a 10 minute cycle for the first example and 15 minutes for the second.

I am not saying that the chart is wrong, I have not seen a heat pump since 1979. I am just asking what else is coming into play here?

George

Oh yea, one more thing. My wife says that I am a well oiled, fine tuned machine. An eating machine.

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Originally posted by a46geo

Brian,

Oh yea, Or Jim Kated.

Jim,

I do not understand how outdoor temperature could affect indoor temperature rise, unless the equipment is staged or augmented using an outdoor sensor to control the process. But in a pure heat pump application, ODT should have no bearing on TR across the coil.

If my indoor return air is 65 degrees Wether it is 40 degrees outdoors or 30 degrees, I should have the same rise across the coil. The only difference ODT makes would be in run time. Perhaps a 10 minute cycle for the first example and 15 minutes for the second.

I am not saying that the chart is wrong, I have not seen a heat pump since 1979. I am just asking what else is coming into play here?

George

Oh yea, one more thing. My wife says that I am a well oiled, fine tuned machine. An eating machine.

Well as I see it, since the coil sizes, fan speeds and compressor speeds are constant, any change in outdoor temperature is going to affect the amount of heat that the refrigerant takes up in the outdoor coil. This change is reflected in the amount of heat that it releases at the indoor coil. Metering devices can help to even out the extremes, but their effect is limited.

Some of the really fancy new systems have two stage compressors, variable speed fans outside and variable speed blowers inside. These super-efficient systems can come closer to the ideal you describe. Since I've only seen a handful of them, I really don't know to what extent they can overcome the effect of changing outdoor temperature.

I've attached another discussion of using the temperature split to evaluate heat pump performance in the heating mode. It has a slightly different chart, but similar enough that either of them should be useful.

I don't know about your prowess as an eating machine, but I was reading over on the "Midwestern Swingers Network Forum," that you're known as a "love machine."

- Jim Katen, Oregon

Download Attachment: icon_word.gif HeatPumpHeating Mode.doc

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Originally posted by a46geo

Jim,

I do not understand how outdoor temperature could affect indoor temperature rise, unless the equipment is staged or augmented using an outdoor sensor to control the process. But in a pure heat pump application, ODT should have no bearing on TR across the coil.

If my indoor return air is 65 degrees Wether it is 40 degrees outdoors or 30 degrees, I should have the same rise across the coil. The only difference ODT makes would be in run time. Perhaps a 10 minute cycle for the first example and 15 minutes for the second.

The key factor is that the the only source for the heat added to refrigerant is the outdoor air. In heating mode the outdoor coil of our heat pump is now an evaporator coil. The refrigerant boils in the outdoor coil because it's heated by the outdoor air passing over the coil. As the OD temp goes down, so does the heat content of the refrigerant. When it get's inside to what is now a condenser coil, there is less heat available to transfer to the indoor air.

It's just like air conditioning in reverse. With an air conditioner, if you measured the temperature split at the OD coil, it would go down as the temperature in the house goes down.

Otherwise, what Katen said. I agree with the article. It's tough to use heating temp rise to evaluate heat pumps. Take the numbers on the chart with a grain of salt. I don't call them defective unless they are way out of whack.

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Originally posted by Jim

Well, I can't speak for George, but really. . . is there a difference? Aren't we all just machines, sinew and blood, bile and breath? Is there not a little of the heat pump in us all? And if so, what is the measure by which we shall be judged? Our "temperature rise" as it were.

Ah, philosophy...how ironic that you should philosophise about whether we are anything more than machines ourselves, given that philosophy is one of the very things which proves that we are, Sir Katen. By which measure shall we be judged? By a great many different ones, in truth. My personal favorite is the one which says you can judge a man by the way he treats those who can do him no good whatsoever (but there are plenty of others). [:-king]

How the hell did this find its way into heat pump discussion? I love it. [:D]

Brian G.

Philosopher, Statesman, BS Artist [:-tophat]

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